Generally, block encoding is a highly-efficient encoding method for upgrading transmission efficiency of a video signal. The block encoding method, which is based on a strong correlation between a picture signal and a sampled value adjacent thereof, accumulates and blocks a predetermined number of samples of picture signals and performs compression in each block. This kind of block encoding method is divided into a transform encoding method which transforms the sampled values within a block for quantization, and a vector quantizing method which quantizes the transformed sample values in each block unit. The transform encoding method is based on the correlation between video signals in minor domains of the screen. During the transform encoding method, pixel values of each minor domain are converted to columns of numerical values. Transform coefficients are obtained from these numerical columns and thus, represent components of the original or predicted error signal ranging from a low frequency to a high frequency.
In this kind of transform encoding method, various transforms may be used for transforming an incoming signal, such as DCT (Discrete Cosine Transform), Hadamard Transform, Haar Transform, KLT (Karhunen-Loeve Transform), BTC (Block Truncation Coding), and the like. The efficiency of these transforms is conventionally expressed in terms of energy packing efficiency (EPE). An EPE is expressed as an energy ratio between a transform energy of M.times.M' units and a transform energy of N.times.N' units, where the M.times.M' units are selected out of N.times.N' units (M or M'&lt;N or N'). The efficiency of a transform having a larger EPE is regarded as excellent. The following is an equation expressing the EPE in a two dimensional method. ##EQU1## where, E[F(u, v)] is the energy level from the transform coefficient F(u, v).
The KLT transform method has the largest EPE of the above noted transform methods, but this kind of transform is impossible to realize through hardware used by the above-noted conventional transform encoding methods. The DCT transform is used as the nearest method to the KLT method for creating a large EPE. However, the EPE characteristic of the DCT method decreases under certain transform conditions. Transforms other than the DCT method, can often provide a better EPE characteristic depending on the incoming signal patterns. For instance, when a picture signal's correlation is less than 0.5, the DST method is more efficient than the DCT method.
As seen by the foregoing, various transform methods have been utilized to improve encoding efficiency. However, when a single transform is used for all incoming signal patterns the EPE decreases, thus preventing the use of an effective correlation method.